Interface SegmentReader.CoreClosedListener

This listener is called only once all SegmentReaders
sharing the same core are closed. At this point it
is safe for apps to evict this reader from any caches
keyed on SegmentReader.getCoreCacheKey(). This is the same
interface that FieldCache uses, internally,
to evict entries.

WARNING: This API is experimental and might change in incompatible ways in the next release.